Backhaul networks connect core network with smaller edge networks, or network elements. In mobile communications networks an example of a backhaul network (mobile backhaul) is a network connecting wireless base stations to their corresponding base station controllers. Using Ethernet for implementing these backhaul networks is becoming ever more popular solution.
Connecting Ethernet links to network devices (e.g. Ethernet switches) can be done via a Small Form-factor Pluggable (SFP) transceiver. Ethernet exists in both optical and electrical formats and SFPs that are present on the market support either electrical or optical Ethernet. Small Form-factor Pluggable transceivers give the flexibility of connecting to network devices electrical or optical Ethernet by using correct SFP, without any change to the network device itself. At one end a SFP comprises an interface for connecting the SFP to a motherboard of a network device, or host device, (e.g. switch). This end is commonly known as unit side. At the other end, also known as line side, the SFP comprises electro-mechanically standardised RJ45 socket for connecting electrical Ethernet cable. If the SFP supports optical Ethernet then instead of RJ45 socket a corresponding optical connector is present. Introduction of SFPs for electrical Ethernet has led to inclusion of an Ethernet PHY (physical layer) into the SFP to pre-process the standard electrical Ethernet signals to the point where they become sufficiently similar to electrical signals converted from optical Ethernet signals. This allows for using identical post processing electronics.
To use Ethernet in mobile backhaul effectively it is necessary to maintain time and frequency synchronisation. The most popular time and frequency synchronisation technologies for Ethernet currently are Precision Time Protocol (PTP) defined in IEEE1588 standard and synchronous Ethernet (SyncE) as specified in ITU-T Rec. G.8261, ITU-T Rec. G.8262 and ITU-T Rec. G.8264 standards.
However, introduction of the processing of Ethernet signals at physical layer in SFPs compromises the Synchronous Ethernet (SyncE) clock chain affecting IEEE1588 standard performance due to long intra-node path before/after timestamp for ingress/egress messages.